new converter topologies (1).ppt - ieee · – three gorges, china (10,000 mw), +/- 600 kv ......

IEEE Southern Alberta Section, Sept. 12, 2011

New Converter Topologies for High-Voltage Dc Converters

Prof. Ani GoleUniversity of Manitoba, Canada


Outline

• Brief History of HVDC Transmission• Conventional HVDC and its Problems• Capacitor Commutated Type Converters• Voltage Sourced Converter Based HVDC

– PWM Based– Multi-level Modular


Originally HVDC was used for Distribution (Edison’s Dc Dynamo) (pre 1900)Disadvantages: Complicated machinery (dc commutator), lack of voltage transformabilityAc overcame these disadvantagesHowever:

Long distance DC transmission is not adversly affected by Transmission Line or Cable inductance/capacitance

HVDC: Brief History


• Why not generate and consume ac but transmit dc?

• Thury (early 1900’s) in France: ~100 km Dc tranmission– Disadvantage: Ac/Dc Converter – motor

generator set• Use of Power Electronic Devices

(Mercury-Arc Valves) made for more efficient Ac/Dc Conversion

HVDC: Brief History…


• First Scheme Based on Modern day concepts:– Gotland (Sweden Mainland-Island) 1954,. Used Grid

Control Mercury Arc Rectifiers. Manufacturer ASEA 100 kV (Monopolar), 20 MW under-sea transmission spanning 96 km.

• First Canadian Scheme:– Vancouver - Vancouver Island, 1968, +/-130 kV , 312

MW, 41 km overheadline, 32 km underwater cable.• Last Mercury Arc Scheme:

– Nelson River Bipole 1 in Manitoba (1800 MW, +/-450 kV)



• First Canadian Scheme:– Vancouver - Vancouver Island, 1968, +/-130 kV , 312

MW, 41 km overheadline, 32 km underwater cable• First Use of Solid-State Thyristors :

– Eel River (New Brunswick-Quebec, Canada) :1972, +/-80 kV, 350MW. Back to back connection between two utilities.

• Large HVDC Systems:– Itaipu (Brazil, Generation: Paraguay/Brazil) +/- 600

kV, 6000 MW, over850 km. Main reason for Dc: Paraguay is 50 Hz, Brazil is 60 Hz.

– Volvograd Dunbas: USSR, 6000 MW?– Three Gorges, China (10,000 MW), +/- 600 kV



• Manitoba:– Nelson River Bipole-I (Radisson-Dorsey) +/-

450 kV, 1800 MW, over 900 km, originally based on Mercury Arc (1972, 1993, 2004)

– Nelson River Bipole -II (Henday-Dorsey): +/-500 kV, 2000 MW, approx. 900 km, Thyristor (1982-88)

– Nelson River Bipole –III (Henday-Riel)– 1400 km? 2200 MW +/- 500 kV



HVDC +/- 500 kV

Manitoba Hydro’s Nelson River HVDC Transmission System: 4 GW over 950 km (approx. 70% of total Manitoba installed generation)

Approx. 40% of MH revenues come from exports

Manitoba Dams are a reservoir that permits power cycling

Revenue generated includes power cycling (day/night)


Many technology revisions


Conventional HVDC Transmission-Advantages

• HVDC Offers many advantages over Ac Transmission– Lower Transmission losses– Smaller rights of way– Asynchronous Connection Between Ac

Networks- improved stability limit– Possibility of Long-distance

underground/underwater cable transmission– …..etc

IEEE Southern Alberta Secion, Sept. 12, 2011

Basics of HVDC LCC Converter OperationDc Converter Building Block: Thyristor


Conventional HVDC: LCC Operation and Limitations:

a) ac voltages

b) dc voltages

c) ac current in phase a

d) Valve T1 voltage

e) Firing Pulse

Id

Vd

- Commutation voltage drop

- Converter Operation is significantly impacted by ac network


• However there are some disadvantages:– The terminating ac networks must provide the

commutation voltage– Require reactive power at the converter which

must vary with loading (i.e. switched filter banks)

– Difficulty in operating into weak ac systems (Short Circuit ratios under 2)

– Generates Ad and Dc side Harmonics

Conventional HVDC Transmission-Limitations


• New converter configurations have been developed to address these issues:

• Capacitor Commutated Configurations– CCC– CSCC

• Voltage Sourced Converter (VSC ) based Configurations– PWM / SHPWM based Converters– Modular Multilevel Converters (MMC)

New HVDC Converter Configurations


Capacitor Commutated Converter

The CCC Uses the voltage across its series capacitors to assist in the commutation process

It can operate into very weak ac networks

The reactive power absorbed by the converter is minimal

Can be operated even with leading power factor


CCC Operation


Reactive Power Requirement


Ac Filter Issues •A low Mvar filter is also sharply tuned and hence subject to detuning with component variations•Solution:

–Contune Filter (inductor can be tuned via bias dc current)–Active Ac Filter


Dc-voltage [kV]

0

200

400

600

800

1000

CCC

Conventional

a)

Inverter ac-voltage [kV]

Id [kA]0,0 0,4 0,8 1,2 1,6 2,0 2,4 2,8

0

100

200

300

400

500Conventional

CCC

b)

c) Maximum Power Curve [MW]

0

200

400

600

800

1000

CCC

Conventional

d)

CCC

ConventionalBasecase

Real extinction angle [degrees]

Id [kA]0,0 0,4 0,8 1,2 1,6 2,0 2,4 2,8

0

10

20

30

40

50

Basecase

CCC Steady State Operating Charecteristics


• The risk of commutation failure is minimized-can operate into very weak ac networks

• The apparent extinction angle (measured w.r.t. converter bus) is small, even negative- hence power factor is near 1.0

• Filter switching can be avoided• Although valves are more expensive, the

converter transformer is cheaper and the valve short circuit current is smaller than for the LCC

• The Series Capacitors do not cause ferroresonance, as they are out of the circuit when converter is blocked

CCC Configuration: Advantages


• The converter cost is slightly larger• The series capacitors must be protected

against overvoltages resulting from overcharging

• The energy storage on the series capacitors negatively impacts the dynamic response in unbalanced conditons (i.e. recovery from l-g faults)

CCC Configuration: Disdvantages


• Garabi Converter Station, Brazil/Argentina

CCC Installations worldwide:

• 2200 MW, +/- 70 kV back to back system connecting 50 Hz and 60 Hz networks

• CCC used because SCMVA can be as low as 2000

• CCC Avoids installation of Synch. Compensator

Courtesy: ABB


Garabi CCC HVDC Station Layout

Courtesy: ABB


Garabi CCC HVDC: Major Components

All Pictures: Courtesy ABB

Outdoor Valves

“Contune” Filters

Series Capacitors


Rapid City, USA, Interconnect

• 200 MW, +/- 12.85 kV

• CCC selected to lower comm. Fail risk due to extremely weak ac networks.

•Sixth in sequence of Back to Back HVDC Stations connecting the Eastern and Western North American Systems


Alternate Topology: CSCC

• Simplifies capacitor arrangement in 12-pulse configurations

• For radial ac feeds, capacitors can be placed in each ac line for accurate control of power in each ac feeder

•Requires only LCC

•Behaviour very similar to CCC

•Series capacitors must be switched to avoid ferroresonance

•Capacitance level can be adjusted as per system conditions


New Approaches to LCC: The GPFC

Filters are between transformer and converter

Uses a Conventional Transformer

Transformer at remote end can be eliminated

Results in reduced cost


Cost Distribution for Converter Station


GPFC-HVDC 12-pulse arrangement

IEEE Southern Alberta Section Sept. 12 2011

• Thyristor Based Converters generally require an ac network to provide commutation voltage

• Hence they are significantly affected by ac system conditions, etc.

• The VSC uses switches that can be turned on as well as turned-off using externally generated commands

• Hence the impact of ac system conditions on performance can be minimized

Voltage Sourced Converter (VSC) Based HVDC


VSC: Basic Operating Principle

VSC Switches are turned on and off on command.


Three Phase Arrangement

IEEE Southern Alberta Section, Sept. 12., 2011

VSC Voltage Magnitude and Phase Control

Pulse Width Modulation

Fundamental freq. component of output follows the desired ‘signal’ reference waveform

Harmonics are pushed to the high (easily filtered) range

Disadvantage:

–Difficult to extend single bridge to High Voltages

–High Switching Losses


VSC: Real and Reactive Power Control

Id* controls the real powerIq* controls the reactive power

Id* is the output of a dc bus capacitor voltage controller


Decoupled Control ensures that an order change of id* does not cause a transient in iq (and vice versa)

See: . Papič, P. Žunko, D. Povh and M. Weinhold, “Basic Control of Unified Power Flow Controller,” IEEE Trans. Power Systems, vol. 12, no. 4, pp. 1734-1739, Nov. 1997.

VSC: Decoupled Control


VSC versus LCC HVDC

+ V

- V

Idc →

LCC HVDC VSC HVDC

Line-commutated Gate-turnoffCurrent Source Voltage SourcedPoorer performance with weak ac systems Less affected by system strength

Cheaper for High Power More expensive, but may be comparable when all aspects are considered

Lower Losses Higher losses (improved by new topologies)

Power direction reversed by voltage reversal

Power direction changed by current reversal

Difficult to use in a dc grid Well suited for dc gridIdeal for dc transmission with overhead lines

Ideal for weak ac systems, cable transmission or dc grids


• Purpose: To Run Compressor Motors for Offshore Gas Extraction

• Gas Pressure from Wells decreases as gas is extracted, hence a compressor is needed to force gas through pipeline

• A conventional precompression project, with gas turbines, would have resulted in annual emissions of some 230,000 tons of CO2 and 230 tons of NOx.

Example of VSC HVDC: Troll Link


• Location: Offshore Norway


One Half of Troll HVDC System


Troll VSC HVDC: Ratings

Main data Rated power 2x40 MW DC voltage ±60 kV AC system voltage 132 kV AC motor voltage 56 kV AC filters Kollsnes: 39’th and 78’th harmonic Troll A: 33’th and 66’th harmonic IGBT valves Valve type Two level Cooling system Water IGBT type 2,5 kV/500 A Cable Type Triple extruded polymer Cross section 300 mm2 Length 4 x 70 km Transformers (Kollsnes only) Type Three-phase, two winding Rated power 52 MVA


Multilevel Modular Converter (MMC)

• PWM converters produce a waveform with high level of higher order harmonics

• Result: High Switching Losses, EMI, Stresses etc.• With High Voltages, Device ratings become an

issue

E/2 R L

E/2

D1

D2

T1

T2

ILVL

Simple Voltage Sourced Inverter


Basic unit of MMC scheme – Submodule

Submodule

T1

T2

D1

D2

C

Tx – IGBT

Dx – Diode

C – Storage Capacitorx = 1,2

T1/D1 conducting

T2/D2 conducting


Device ON V0

SW1 Vc

SW2 0

Each submodule acts as a controllable voltage source.

MMC Submodule

Submodule

Ic

Vc -

+

SW1

V0

I0

SW2

Control States of a Sub-module


MMC Topology

SMi

+Vd

-Vd

MMC basic scheme

B C

SM1

SM2

SMn

SM1

SM2

SMn

T1

T2

D1

D2

C


Introduction – MMC Topology+Vd

-Vd

MMC basic scheme

B C

SM1

SM2

SMn

SM1

SM2

SMn

VA

VA

Phase Voltage (n = 10)

t

Vd

-Vd

VA

Phase Voltage (n = 10)

t

Vd

-Vd


MMC Controls• Reference Waveform is

quantized to determine switching instants

• Special algorithms for Capacitor voltage balancing and ensuring sharing of module duty

• Higher level controls identical to other VSC topologies (i.e. decoupled id/iq control etc.)


Trans-Bay HVDC Project

• Purpose: – Congestion Relief– Improvement of security of supply– Retirement of Generation in San Francisco Area

• Customer Trans Bay Cable, LLC • Location Pittsburg, California, and San Francisco,

California • Power Rating 400 MW • Voltage levels ± 200 kV DC,

230 kV /138 kV, 60 Hz • Type of plant 85 km HVDC PLUS

submarine cable • Type of Thyristor IGBT


Transbay Cable (San Francisco-Oakland)

Courtesy: Siemens


Courtesy: Siemens

IEEE Southern Alberta Section Sept. 12 2011Courtesy: Siemens


HVDC Supergrids?• VSC Converters enable construction of HVDC Grids• Reduced Losses• Increased power capacity per line/cable vs. AC• Underground/Underwater or reduced rights of ways

imply:– lesser right of way limitations, – lower visual impact and lower EM fields

• Stabilized AC & DC grid operation – AC networks can be asynchronous

• Applicable for Harnessing Multiple off-shore windfarms


HVDC Transmission Technology is evolving to adapt to the change in attitudes about energy

The barriers on conventional LCC HVDC imposed by the ac system conditions are being overcome

CCC Technology extends the range of thyristor based converters

VSC technology is promising - less influenced by the ac network

Recent innovations such as the MMC are reducing losses and making VSC technology very attractive

The future is bright - radical changes in the power network, such as dc grids are on the horizon

Concluding Remarks

new converter topologies (1).ppt - ieee · – three gorges, china (10,000 mw), +/- 600 kv ......

Documents